1. Field of the Invention
The present invention relates to a TDD (Time Division Duplex)-based mobile communication system and, more particularly, to an apparatus and method for synchronizing the transmission of uplink signals and reception of downlink signals by a terminal in a mobile communication system.
2. Description of the Related Art
TDD is a duplexing technique which temporally divides a radio channel so that one portion of a frame period is allocated for uplink transmission and the remaining portion of the frame period is allocated for downlink transmission. TDD is a third-generation mobile communication system defined by UTRA (UMTS Terrestrial Radio Access) standards of an ETSI (European Telecommunications Standards Institute) UMTS (Universal Mobile Telecommunications Systems).
In a TDD communication system, transmission and reception of wireless signals are made in a common frequency band. Since the same frequency is used for uplink transmissions and downlink transmissions in a TDD system, the transmissions to and from a terminal are carried according to time slots previously set by a base station.
In a frequency division duplex (FDD) communication system, because radio bands for transmission and reception are separated, uplink/downlink synchronization does not need to be adjusted. As long as these bands are within a defined range, transmission and reception channels can be easily obtained.
In comparison, accurate uplink/downlink synchronization is a requisite for a TDD system. Unless synchronization is maintained, communications are not possible. Additionally, multimedia communications such as a voice or image, for which an initial synchronization must be acquired, requires even more precise uplink/downlink synchronization.
Referring to FIG. 1, a schematic block diagram of a related art TDD terminal is illustrated. As shown in FIG. 1, the related art TDD terminal 1 includes a transmitter 60, a receiver 70, a TDD switch 40, and a digital base-band modem 10 (hereinafter referred to as a ‘modem’).
The transmitter 60 further includes a filter 20, a digital/analog converter 21, an intermediate frequency (IF) signal processor 22, and an RF signal processor 23. The receiver 70 further includes an RF signal processor 33, an IF signal processor 32, an A/D converter 31 and a filter 30. The transmitter 60 converts a data signal into a radio frequency (RF) signal. The receiver 70 converts a received RF signal to a signal that the modem can process. The TDD switch 40 performs a switching operation to alternately connect an antenna 85 to the receiver 70 or to the transmitter 60. The modem 10 further includes a software controller 11 and controls the TDD switch 40.
The TDD switch 40 performs a switching operation to connect the antenna 85 to the RF signal processor 33 of the receiver 70 for receiving downlink signals (downlink switching) or to the RF signal processor 23 of the transmitter 60 for transmitting uplink signals (uplink switching). The switching is done in such a manner that a downlink time slot and an uplink time slot do not overlap.
The uplink/downlink synchronization process of the terminal 1 is performed when the antenna 85 receives an RF downlink signal. When the TDD switch 40 connects the antenna 85 to the receiver 70 (downlink-switching), an RF signal collected by the antenna is transferred to the RF signal processor 23 and then to the IF signal processor 32. The RF signal is converted by the IF signal processor 32 into an IF signal which passes through the A/D converter 31 and filter 30 to the modem 10.
The modem 10 demodulates the received signal and detects a boundary of the downlink time slot, which is a switching point for downlink transmission. The modem 10 then determines a switching time of the TDD switch 40, taking into consideration a signal processing delay time defined by the communication system.
The signal processing delay time is the time required for elements constituting the transmitter 60 and receiver 70 to process signals indiscriminately transferred to a terminal from a base station in the system. Since the signal processing delay time is a fixed value, it may be much different from the actual delay times of elements provided in any particular terminal 1.
When the switching point is determined, the modem 10 transmits a transmission signal to the transmitter 60 for uplink transmission. According to the determined switching point, the modem 10 controls the switching operation of the TDD switch 40. Once determined, the operation of the TDD switch 40 is maintained according to the determined switching point. However, it is not known whether the TDD switch 40 operates properly.
Attempts to lower a production cost of terminals 1 have involved implementing the modem 10 in software. With a software modem 10, however, it is difficult to precisely control positions of time slots according to the software clock. Furthermore, once synchronization is obtained, the positions of the time slots may change and degraded performance may result. Because a transmission-determined point, at which uplink transmission is made, and the point at which data is actually transmitted change according to the software clock, a software-based modem 10 cannot precisely control the time slots.
If the modem 10 is based on a hardware platform, variation in the time required for a signal from the modem 10 to reach the RF signal processor 23 of the transmitter 60 is small, so the switching operation may be performed accurately. However the production costs are higher than for a software-based modem 10.
Regardless of whether the modem 10 is hardware-based or software-based, there are other drawbacks to the modem 10 of the related art. First, although the modem 10 can precisely adjust the time slot boundary of the receiver 70 by using a synchronous signal provided from the terminal 1 or from a base station (not shown), it still cannot be determined whether the transmitter 60 is accurately synchronized. Second, although synchronization of a transmission channel may be gradually adjusted, a base station generally processes communications of several terminals 1. Therefore, inaccurate transmission of one terminal 1 may affect communications of other terminals, for example if time slots of one terminal that are transmitted without being synchronized for transmission intrude upon time slots allocated to other terminals.
Due to the internal calculation process of the modem 10, data transmission is not instantaneously performed and, in most cases, the determined transmission point does not correspond to the time point at which data is actually transmitted. Therefore, it is virtually impossible for the TDD switch 40 to precisely operate so that the uplink time slot may not intrude upon the boundary of the downlink time slot.
Therefore, there is a need for an apparatus and method that performs switching between the reception of downlink signals and transmission of uplink signals in a terminal of a mobile communication system such that the uplink time slot does not intrude upon the boundary of the downlink time slot. The present invention addresses this and other needs.